This invention relates generally to demodulation of signals in detection systems.
One of the major problems in active transmit-receive detection systems is that the sensors that are supposed to pick up the response of the return signal from an object or secondary source, respond to the direct signal from the transmitter. This is a problem for several reasons. In most systems the transmitter signal is much larger than the return signal. In some systems this ratio can be over one million to one. Furthermore, in most systems the receive sensors are located physically closer to the transmitter than to the object that gives the return. Due to these factors, the desired sensor signal (the signal due to the return) is masked by the much larger sensor response to the direct signal. System designers spend great efforts to reduce the direct pickup by the sensors. In many cases the limiting sensitivity of a system is set by the system's ability to reduce the direct pickup of the sensors. This is especially true in continuous mode systems (systems that continuously transmit a signal).
To avoid this problem, systems were developed that transmitted relatively short pulses. The sensors were then set up to look only at the returns that came after the transmitter had stopped producing the pulse. FIG. 1. shows a typical transmitted pulse signal and its associated return signal. FIG. 2. shows the portion of the return signal that is recorded and analyzed by the pulsed system. In this manner the systems could transmit a relatively higher power pulse, albeit for a short period of time, that would not interfere with the return signal on the sensors. Since the pulse transmit times are relatively short the total transmitted energy is small. In these types of systems the elimination of the direct transmit signal at the sensor is related to the time it takes to turn off the transmitter and the waiting time between the pulse end and the time the sensor starts responding to the return signal. The nature of the laws of physics means that no real system can stop transmitting a signal instantaneously. This means that the system must wait until the direct signal at the sensor falls below a given level before it can use the sensor for detecting the return. These factors along with the response of the object to the transmitted signal govern the system's responsivity to the object.